1. Field of the Art
This invention relates to a method for maintaining the activity of zeolite catalysts in the process for producing methylamines from methanol and ammonia with the use of such catalysts. More particularly, this invention relates to a method for maintaining the activity of zeolite catalysts by suppressing contamination of the reaction system with impurities of a specific kind, which is unavoidable under ordinary conditions.
2. Background Art
In general, methylamines are prepared from methanol and ammonia under the conditions of a pressure of 5 to 40 atm and a reaction temperature of 350.degree. to 450.degree. C., using a solid acid catalyst such as silica-alumina. Three types of methylamines, i.e., mono-, di- and tri-methylamine (hereinafter abbreviated as MMA, DMA and TMA) are formed depending upon the number of methyl groups bonded to the nitrogen atom of ammonia. Such methylamines are all useful as intermediates for various solvents, pharmaceuticals, organic synthesis, dyeing aids, surfactants and so on. However, demand for DMA for use primarily as the starting material of dimethylformamide in large quantities is much greater, say, about ten times the demand for MMA and TMA in Japan. In the presence of such solid acid catalyst, however, the composition of the product is thermodynamically determined and nearly equal amounts of MMA, DMA and TMA are formed at the same time under ordinary conditions. Hence, large proportions of the formed MMA and TMA are returned to the reaction system after separation and reused as the starting materials. Furthermore, in view of equilibrium, there is a need for using a largely excessive amount of ammonia for the purpose of promoting the formation of DMA. Separation and recycling of such excessive methylamines and unreacted ammonia cause the installation to increase in size and the process to consume a large quantity of energy. For the details of such a process, see for example, "Hydrocarbon Processing", 1981, November 1985.
Besides the so-called "conventional catalytic process" as described above in which the reaction involved is governed by thermodynamic equilibrium, there has been recently developed a process for obtaining an unequilibratory reaction product containing predominantly primary or secondary amine (MMA or DMA) by making use of the shape selectivity of zeolite catalysts. In this process, a zeolite (crystalline alumino silicate) having a pore opening diameter of a size intermediate between the critical molecular sizes of primary or secondary amine and tertiary amine is used as a catalyst and the molecules of tertiary amine are prevented from diffusing out of the pores, whereby primary or secondary amine is obtained selectively. With this process, DMA may be selectively produced independently of thermodynamic equilibrium, providing various merits such as considerable decrease in the amounts of MMA and TMA to be recycled and excessive ammonia, reduction in process scale, and energy saving. Specific processes proposed thus far utilizing such zeolite catalysts include a process for predominantly obtaining MMA with the use of ZSM-5 or ZSM-21 (U.S. Pat. No. 4,082,805), a process for predominantly obtaining MMA with the use of mordenite, ferrierite, erionite or clinoputilolite (Japanese Patent Laid-Open Publication No. 56-113747) and a process for predominantly obtaining MMA with the use of levynite (EP107457). Specific processes proposed for predominantly obtaining DMA include those using low-binder A zeolite (Japanese Patent Laid-Open Publication No. 58-69846), Fu-1 (Japanese Patent Laid-Open Publication No. 54-148708), mordenite (Japanese Patent Laid-Open Publication No. 58-49340), mordenite or clinoputilolite (Japanese Patent Laid-Open Publication Nos. 57-4169444 and 59-21005) and Rho, ZK-5, chabasite or erionite (Japanese Patent Laid-Open Publication No. 61-254256).
Although only a few reports are available on the process for the production of methylamines using catalysts giving such unequilibratory compositions, its general aspect is described in Fujita et al, "Catalysts", vol. 129, No. 4 (1987). In such a process, the selectivity for DMA is improved by about twice in comparison with that of the conventional process, i.e., the thermodynamic equilibrium process, whereas the selectivity for TMA is reduced to about 1/5. However, since the zeolite catalyst used has an extremely limited selectivity for TMA and TMA returned to the reaction system shows no substantial reactivity over this catalyst, the productivity of TMA (relative to DMA) is limited to a very narrow range in the presence of such catalyst. In order to solve this problem and with a view to making it possible to produce each methylamine at a wider range of given ratios, it has been proposed to use the conventional equilibrium-governed type of catalyst (non-zeolitic catalyst) together with a zeolite catalyst in parallel or series (Japanese Patent Laid-Open Publication No. 57-169445).
One of the characteristic features of processes for producing methylamines using zeolite catalysts is that the reaction is carried out at a temperature lower than that applied in the prior art. This is because the effect of molecular shape selectivity increases at a low temperature, and the amount of the by-product coke formed decreases with a decrease in the reaction temperature, thus leading to an increase in the service life of the catalyst. Another .feature is that methanol is not rendered to be reacted completely, unlike the conventional processes, with the conversion of methanol being usually limited to 80 to 98%. This is because the effect of molecular shape selectivity drops drastically at a conversion exceeding 98%. In most cases, therefore, the unreacted methanol is separated, recovered and recycled to the reaction system for reuse.
In general, coke is formed over a zeolite catalyst in a relatively large amount and tends to have a sharp influence on the catalytic activity of zeolite. In particular, it has been found that a zeolite having an one-dimensional pore structure such as mordenite is apt to be deactivated by coke. In the commercial production of methylamine with the use of a zeolite catalyst, the service life of the catalyst is in general shorter than about two or three months even when a critical low temperature of 300.degree. C. or lower is applied to restraint the formation of coke. This makes the efficient use of zeolite catalysts extremely difficult.
Until now, various methods have been proposed with a view to controlling the amount of coke formed over zeolite catalysts or reduce its influence. For instance, studies have been made on methods relying upon the introduction of a third substance such as Pd or P [Ono, "KAGAKU TO KOGYO (Chemistry and Industry)", 38, 100 (1985)], the control of the acidic nature (acid strength distribution) of catalysts [Sawa et al, the 58th Forum on Catalysts, Proceedings (A)], the selective poisoning of the outer surface activity of catalysts [Dejaifve et al., J. Catal. 70, 123 (1981)], the adjustment of the size of zeolite crystals [Sugimoto et al., "Shokubai (Catalysts)", Vol. 25, 13p (1983)] and the adjustment of the hydrophilic and hydrophobic nature of catalysts [Okazaki et al., "Shokubai", Vol. 25, 4p (1983)]. However, these methods fail to provide an essential solution to the problem of the deactivation of zeolite catalysts by coke. In-practice, a catalyst-regenerating arrangement is required to be incorporated in the processes using zeolite catalysts so as to continuously or frequently regenerate the zeolite catalysts. In the process for the production of methylamines in which the amount of gases passing through a reaction tower is considerably large relative to the output for the aforesaid reasons, however, such regular and frequent regeneration of the catalysts are disadvantageous in view of production costs and productivity. It is thus required that plants be continuously operated over an extended period without any regeneration of catalysts.